The present invention relates to systems and methods for recovering ethylene, ethane, and heavier hydrocarbons from natural gases and other gases, e.g. refinery gases, and in a further embodiment relates to methods and structures for recovering ethylene, ethane, and heavier hydrocarbon components from natural gases and other gases using a cryogenic expansion process with a relatively low expansion ratio across the expander.
Cryogenic expansion processes have been well recognized and employed on a large scale for hydrocarbon liquids recovery since the turbo-expander was first introduced to gas processing in the late 1960s. It has become the preferred process for high ethane recovery with or without the aid of external refrigeration depending upon the composition (richness) of the gas. In a conventional turbo-expander process, the feed gas at elevated pressure is pre-cooled and partially condensed by heat exchange with other process streams and/or external propane refrigeration. The condensed liquid with less volatile components is then separated and fed to a fractionation column (e.g., a demethanizer), operated at medium or low pressure, to recover the heavy hydrocarbon constituents desired. The remaining non-condensed vapor portion is subjected to turbo-expansion to a lower pressure, resulting in further cooling and additional liquid condensation. With the expander discharge pressure typically the same as the demethanizer pressure, the resultant two-phase stream is fed to the top section of the demethanizer with the cold liquids acting as the top reflux to enhance recovery of heavier hydrocarbon components. The remaining vapor combines with the column overhead as a residue gas, which is then recompressed to pipeline pressure after being heated to recover available refrigeration.
Because the demethanizer operated as described above acts mainly as a stripping column, the expander discharge vapor leaving the column overhead that is not subject to rectification still contains a significant amount of heavy components. These components could be further recovered if they were brought to a lower temperature, or subject to a rectification step. The lower temperature option can be achieved by a higher expansion ratio and/or a lower column pressure, but the compression horsepower would have to be too high to be economical. Ongoing efforts attempting to achieve a higher liquid recovery have mostly concentrated on the addition of a rectification section and how to effectively increase or provide a colder and leaner reflux stream to the expanded vapor. Many patents exist pertaining to a better and improved design for separating ethane and heavier components from a hydrocarbon-containing feed gas stream.
U.S. Pat. No. 4,140,504 describes methods to improve liquid recovery in a typical cryogenic expansion process by adding a rectification section to the expander discharge vapor, and using the partially condensed liquid as the reflux after it is further cooled and expanded to the top of the rectification section. U.S. Pat. No. 4,251,249 adds a separator at expander discharge, separates liquid from the expanded two phase stream, and sends the liquid to column for further processing. The separated vapor provides refrigeration in a reflux condenser to minimize the loss of heavy components in the overhead vapor stream. In yet another approach, e.g. U.S. Pat. No. 5,566,554, the partially condensed liquid is preheated and expanded to a second separator at an intermediate pressure to yield a vapor stream preferably comprising lighter hydrocarbon components. This leaner stream returns to the demethanizer top as an enhanced reflux after being condensed again and subcooled. The reflux stream so generated is rather limited, and the heavy components not recovered are still substantial.
The most recognized approach for high ethane recovery, perhaps, is the split-vapor process as disclosed in U.S. Pat. Nos. 4,157,904 and 4,278,457. In these patents, the non-condensed vapor is split into two portions with the majority one, typically about 65%-70%, passing through a turbo-expander as usual, while the remaining portion being substantially subcooled and introduced to the demethanizer near the top. This higher and colder reflux flow permits an improved ethane recovery at a higher column pressure, thereby reducing recompression horsepower requirements, in spite of less flow being expanded via the turbo-expander. It also provides an advantage in reducing the risk of CO2 freezing in the demethanizer. The achievable recovery level in these processes, however, is ultimately limited by the composition of the vapor stream used for the top reflux due to equilibrium constraints. Ethane recovery up to 90% is achievable when the expansion ratio is high, typically in excess of 2.5.
The use of a leaner reflux is an attempt to overcome the aforementioned deficiency. One approach is to cool the split vapor stream half way through and expand it to an intermediate pressure, causing partial condensation. The condensed liquid comprising less volatile components is separated in a separator and fed to the demethanizer above the feed from the turbo-expander discharge as the mid-reflux. The leaner vapor so generated is further cooled to substantial condensation and used as top reflux. U.S. Pat. No. 4,519,824 is a typical example. U.S. Pat. No. 5,555,748 further improves this process by cooling the separated liquid prior to entering the demethanizer as the mid-reflux.
A substantially ethane-free reflux has been introduced in some processes, which permits essentially total recovery of ethane and heavier components from a hydrocarbon containing feed stream. These processes recycle a portion of the residue gas stream as the top reflux after being condensed and deeply subcooled. Because the residue gas contains the least amount of ethane in the entire process, ethane recovery in excess of 98% is achievable by providing more and leaner reflux from recycle of a significant amount of residue gas. It should be noted that it is the liquid reflux in contact with, providing refrigeration to, and promoting condensation of the uprising heavy component vapor to enhance liquid recovery. Therefore, the recycle of residue gas must be recompressed to a much higher pressure with penalty on compression horsepower to enable its total condensation.
U.S. Pat. Nos. 4,851,020 and 4,889,545 utilize the cold residue gas from the demethanizer overhead as the recycle stream. This process requires a compressor operating at a cryogenic temperature. Warm residue gas taken from the residue gas compressor, eliminating the need of a dedicated compressor, is disclosed in U.S. Pat. Nos. 4,687,499 and 5,568,737. An alternate arrangement with a recycle compressor which is required for a low residue gas pressure scenario and/or permits optimal pressure of recycle residue gas is also presented in U.S. Pat. No. 5,568,737.
To enhance ethane and NGL recovery efficiency, the aforementioned prior arts typically involve generating a colder and leaner reflux stream for the top rectification section of the demethanizer and requiring the turbo expander to operate at a high expansion ratio. In the case of a low feed gas pressure, the pressure of the feed gas has to be raised so that a portion of the gas can be liquefied and fed to the demethanizer as a reflux via the use of cold residue gas as a typical cooling medium. Raising the inlet gas pressure also permits a high expansion ratio across the expander. This option leads to a higher horsepower requirement for the front end compression. Alternately, the demethanizer can be operated at a reduced pressure. However, it leads to a higher recompression horsepower or a possibility of CO2 freezing when the feed gas contains a sufficient amount of CO2. In both cases, compression power has been applied to the total flow either at the front-end (i.e. feed gas) or the back-end (i.e. residue gas) to promote partial condensation of feed gas as a demethanizer reflux and to gain the expander refrigeration, which is generally not the most efficient approach in most cases. In some cases, the equipment for the inlet gas cooling is not designed with a high enough design pressure for a retrofit to an existing facility.
Accordingly, it is an object of the present invention to provide a process for separating components of a feed gas containing methane and heavier hydrocarbons which maximizes ethane recovery but does not require appreciable increases in capital and operating costs.
In carrying out these and other objects of the invention, there is provided, in the broadest sense, a process for cryogenically recovering components of hydrocarbon-containing feed gas in a distillation column, e.g. a cryogenic distillation column such as a demethanizer, in which the main reflux to the demethanizer is provided by compressing and condensing only a slip stream of the feed gas. A reflux compressor compresses the slipstream of the feed gas to a pressure suitable for condensation. Thus the compression power is utilized in the areas where it is needed the most, namely the reflux streams for the demethanizer. Thus, this method avoids unnecessary compression of the whole feed gas stream, and hence avoids waste of horsepower (due to inefficiencies in the compression and expansion process). Shortage in the refrigeration, if any, can be effectively supplemented by either the enhanced stripping gas scheme incorporated with this invention, or the external refrigeration.
In one form of the present invention, the feed gas is split into two streams. The smaller slip stream that has to be used as a reflux, typically ranging from 20% to 40% of total feed gas flow, is compressed by the reflux compressor and is cooled and fed to a cold separator. The use of a cold separator is only optional and is typically recommended when the feed gas contains heavier constituents, such as aromatic compounds, which could potentially freeze up at cryogenic temperatures. The main portion of feed gas is cooled to partial condensation in the inlet heat exchangers and fed to the expander inlet separator for separating condensed liquid components. The separated liquid portion is expanded and fed to the demethanizer. The vapor portion from the expander inlet separator is typically expanded via a work-generating expander and then fed to the demethanizer. In this embodiment the need for refrigeration, if any, to cool the inlet gas is provided by either the enhanced stripping gas scheme or by external refrigeration.
In another form of the methods of the present invention, supplemental refrigeration needed for the inlet gas cooling, if any, is provided by external refrigeration, such as propane.
In yet another form of the methods of the present invention, the inlet feed gas is cooled and partially condensed in the inlet heat exchangers. The partially condensed gas is separated in an expander inlet separator. A slipstream of the vapor portion from the expander inlet separator is compressed, condensed and used as a main reflux in the demethanizer. Thus as compared to the above embodiment, in this scheme the reflux compressor compresses the vapors from the expander inlet separator instead of the feed gas. Since the vapor from the expander inlet separator will be leaner than the inlet feed gas there might be additional advantages associated with this scheme.